chore(comments): drop epic, sprint and review-round ids from the code

The code cited its plan instead of saying why: `E28 review J4`, `(R6 S3b)`, `S1 justified…`,
`deep-review round 5`. That plan lives in the private ops repo, so for every reader of this one the
citation was noise, and a sentence whose subject was the plan step ("round 2 showed…") hid its
lesson behind an id. Each one is gone and the "why" kept: a bare citation is deleted, a sentence
carrying a lesson is reworded around it ("a later attempt showed…").

Measured (`scripts/audit_metrics.py`, product code): 652 lines in 270 files carried an epic code by
the definition the next commit holds (507 / 241 by the old, two-digit-only one) -> 0. Across all
code, tests, scripts and configs: ~1100 epic-code lines plus ~320 sprint/round ids.

Behaviour unchanged, checked mechanically: every changed .py file has the same AST once docstrings
are blanked; css/sh/toml/yml/json files are identical once comments are stripped; the only
non-comment changes in TS/JS are test titles, one description string in a contract table, and five
ESLint messages, whose text is all the gates snapshot diff holds. Not touched: Markdown (docs and
the changelog keep their history), Alembic migrations, TV episode notation (`S2 · E5`), DESIGN.md
ids (`C1`, `B4`, `S3h`), tracker ids (`SEC-16`, `DX-17`).

One stale claim fixed on the way: test_hls_reaper's docstring said the player checkpoints "every
10 s whether or not it is playing", which plex/stream.py calls wrong; it now says what the code does.

frontend/src/components/OnboardingWizard.tsx

@@ -117,13 +117,13 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
 
   // A layer in the shared stack, so Escape closes the wizard only when it is on top: a raw document
   // listener ran BEFORE the error dialog's (a sync that failed inside the wizard opens one), and one
-  // Esc closed the error AND ended onboarding, skipping the write-scope step (deep-review round 4).
+  // Esc closed the error AND ended onboarding, skipping the write-scope step.
   const layer = useLayer(true, close);
 
   // The rest of the dialog contract. This is a modal overlay in every way that matters to a user, but
   // it renders its own shell (it is not a titled dialog with a header row), so Escape ordering was all
   // it had: focus started on <body>, nothing announced a dialog, and Tab past the last control walked
-  // into the app behind the scrim — where the focus ring is invisible (deep-review round 5). The trap
+  // into the app behind the scrim — where the focus ring is invisible. The trap
   // and the roles come from the same place `ui/Modal` gets them, not from a second hand-written copy.
   const panelRef = useRef<HTMLDivElement>(null);
   const rootRef = useRef<HTMLDivElement>(null);
@@ -136,7 +136,7 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
       if (e.key !== "Tab" || layer.blockingAbove()) return;
       const panel = panelRef.current;
       // The SAME three arguments `ui/Modal` passes, for the same two reasons — and this call had
-      // neither of them (round 6, on the commit that extracted the helper so the two shells could
+      // neither of them (on the commit that extracted the helper so the two shells could
       // not disagree). Scope is the whole dialog, so the toast host beside the panel stays in the
       // Tab cycle (a sticky connection notice's ✕ was unreachable from the keyboard); `container`
       // is the tabIndex=-1 panel, because `panel.contains(panel)` is true, so focus sitting ON the
@@ -151,7 +151,7 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
   // Are we currently importing (or about to)? Show a progress screen instead of the
   // "you're connected" step until the first import settles. Once triggered, the mutation itself says
   // which (pending / success / error) — the trigger ref is the effect's once-only latch, never read
-  // here (a ref read in render made the React Compiler skip the wizard, E32).
+  // here (a ref read in render made the React Compiler skip the wizard).
   const importing =
     me.can_read &&
     !importSubs.isError &&